The present invention relates to a secondary air supply system for an internal combustion engine, which is used to oxidize unburnt exhaust gas components emitted from the engine. More particularly, the invention relates to a control valve capable of not only supplying required amount of secondary air to an exhaust pipe so as to oxidize the unburnt exhaust gas components emitted from the engine during normal steady running and acceleration operations of the engine but also releasing almost all of air from an air pump driven by the engine to the atmosphere during deceleration operation of the engine so as to prevent afterburning phenomena.
In a conventional secondary air supply system for an engine, there have been provided individually two kinds of separate control valves, one of which is an air bypass valve (or an anti-after-buring valve) for bypassing or releasing almost all of air from an air pump to the atmosphere for several seconds during engine deceleration so as to prevent afterburning phenomena and the other of which is a relief valve for maintaining the pressure of air from the air pump below a predetermined value so as to ensure a long life-time of the air pump.
The conventional system just described, however, during operations where a large amount of secondary air is required, such as acceleration operation, runs short of secondary air to be supplied to the exhaust pipe since a large amount of air from the air pump is discharged into the atmosphere through the relief valve in spite of the fact that the air pump is pumping out the sufficient amount of the secondary air.
A modification has been also proposed in which the relief valve is constructed within the air bypass valve, however it is disadvantageous in that the modification is high in cost and complicate the construction because the valve bodies of the relief valve and air bypass valve are still separately made in such a way that those valve bodies operate independently.